1. Field of the Invention
The invention relates generally to fracture and wear resistant rock bits. More specifically, the invention relates to compositions for inserts used on rock bits which enhance the useful life of the bit.
2. Background Art
Drill bits used to drill wellbores through earth formations generally are made within one of two broad categories of bit structures. Drill bits in the first category are generally known as “fixed cutter” or “drag” bits, which usually include a bit body formed from steel or another high strength material and a plurality of cutting elements disposed at selected positions about the bit body. The cutting elements may be formed from any one or combination of hard or superhard materials, including, for example, natural or synthetic diamond, boron nitride, and tungsten carbide.
Drill bits of the second category are typically referred to as “roller cone” bits, which include a bit body having one or more roller cones rotatably mounted to the bit body. The bit body is typically formed from steel or another high strength material. The roller cones are also typically formed from steel or other high strength material and include a plurality of cutting elements disposed at selected positions about the cones. The cutting elements may be formed from the same base material as is the cone. These bits are typically referred to as “milled tooth” bits. Other roller cone bits include “insert” cutting elements that are press (interference) fit into holes formed and/or machined into the roller cones. The inserts may be formed from, for example, tungsten carbide, natural or synthetic diamond, boron nitride, or any one or combination of hard or superhard materials.
Breakage or wear of the inserts, among other factors, limits the longitivity of a drill bit. Inserts used with a rock bit are generally subjected to high wear loads from contact with a borehole wall, as well as high stresses due to bending and impact loads from contact with a borehole bottom. The high wear loads can also cause thermal fatigue in the inserts, which initiates surface cracks on the inserts. These cracks are further propagated by a mechanical fatigue mechanism that is caused by the cyclical bending stresses and/or impact loads applied to the inserts. Fatigue cracks may result in chipping, breakage and failure of inserts.
Inserts that cut the corner of a borehole bottom, such as gage inserts are subject to a significant amount of thermal fatigue. Thermal fatigue is caused by heat generated on the gage side of an insert by friction when the insert engages the borehole wall and slides into a bottom-most crushing position. When the insert rotates away from the bottom, it is quickly cooled by the surrounding circulating fluid. Repetitive heating and cooling of the insert initiates cracking on the outer surface of the insert. Thermal fatigue cracks then propagate through the body of the insert when the crest of the insert contacts the borehole bottom (because of the high contact stresses). The time required to progress from heat checking, to chipping, and eventually to broken inserts depends upon formation type, rotational speed of their bit, and applied weight on bit, among other factors.
The interior rows are also subject to thermal fatigue caused by scraping the borehole bottom. The amount of scraping varies from row to row and is influenced by bit offset and cone to bit speed ratio.
Cemented tungsten carbide generally refers to tungsten carbide (WC) particles dispersed in a binder metal matrix, such as iron, nickel, or cobalt. Tungsten carbide in a cobalt matrix is the most common form of cemented tungsten carbide, which is further classified by grades based on the grain size of WC and the cobalt content.
Tungsten carbide grades are primarily made in consideration of two factors that influence the lifetime of a tungsten carbide insert: wear resistance and toughness. As a result, inserts known in the art are generally formed of cemented tungsten carbide with average grain sizes about less than 3 um as measured by ASTM E-112 method, cobalt contents in the range of about 6-16% by weight and hardness in the range of about 86 Ra to 91 Ra.
For a WC/Co system, it is typically observed that the wear resistance increases as the grain size of tungsten carbide or the cobalt content decreases. On the other hand, the fracture toughness increases with larger grains of tungsten carbide and greater percentages of cobalt. Thus, fracture toughness and wear resistance (i.e., hardness) tend to be inversely related: as the grain size or the cobalt content is decreased to improve the wear resistance of a specimen, its fracture toughness will decrease, and vice versa.
Due to this inverse relationship between fracture toughness and wear resistance (i.e., hardness), the grain size of tungsten carbide and the cobalt content are selected to obtain desired wear resistance and toughness. For example, a higher cobalt content and larger WC grains are used when a higher toughness is required, whereas a lower cobalt content and smaller WC grains are used when a better wear resistance is desired.